Animal manure management practices, principally in regions where there is a surplus of manure are often detrimental to the environment and also represent a potential hazard to human and animal health. Animal manures can produce strong odours, encourage fly breeding, induce weed problems and pollute air, soil and water. For example, in some areas of Canada, the drinking water source is polluted and water bodies cannot be used for recreational purposes due to manure contamination. The affected communities are expecting changes in manure management from the farm industry. The National Workshop on Land Application of Animal Manure, CARC (1991), recommended innovative research that would allow farmers to adopt sustainable and environmentally sound agricultural practices where animal manure is integrated into the overall production systems. It was further recommended that economical processes to stabilize, deodorize, recover energy and add value to animal manure be developed.
Conventional anaerobic digestion of animal manure in farm scale digesters was attempted at several locations across Canada during 1975-1985. It was not successful for several reasons (Van Die, 1987) as follows: 1) The digesters were designed to operate at mesophilic (35.degree. C.) or thermophilic (60.degree. C.) temperatures. Because of prolonged sub-freezing winter temperatures in parts of North America, digesters operating at these temperatures during the winter used not only most of the gas they produced but sometimes required supplementary heating to maintain the digester temperature. For example, in a full scale anaerobic digester for cattle manure in Southern Ontario, more energy was required to run the digester in winter months than the energy generated in the biogas produced. 2) The anaerobic digesters were not cost effective because they were designed to produce electricity which made them even more capital intensive. 3) The digesters were not practical for farm use because their control and maintenance required skilled operators, increased labour input, daily supervision and sometimes changes in farm operational procedures. 4) The digesters were difficult to control and had poor stability because they were pushed to the limit to achieve maximum gas production.
Anaerobic digestion of municipal waste water and animal manures at low (psychrophilic) temperature has been reported in previous studies (O'Rourke, 1968; Stevens and Schulte, 1977; Ke-Xin and Nian-Gua, 1980; Wellinger and Kaufmann, 1982; Chandler et al., 1983; Cullimore et al., 1985; Lo and Liao, 1986; Sutter and Wellinger, 1987; Balsari and Bozza, 1988; and Safley and Westerman, 1992, 1994). Most of these studies were aimed at biogas production while little consideration was given to odour reduction, waste stabilization or increases in fertilizer value or plant nutrient availability. There was a wide variation in the reported experimental results. Some studies were successful in producing methane at temperatures below 20.degree. C. while others were not. The information provided in the above reports is inadequate to provide possible reasons for these discrepancies. In most of these studies the solids were separated from the liquid or the slurry solids content was very low (less than 2%) compared to the typical solids content of manure slurry at Canadian farms. It is unlikely that farmers would dilute manure slurry for anaerobic digestion because it would require larger storage facilities and increase substantially the volume of liquid manure to spread on the land. Furthermore, farmers are not interested in separating the liquid and solid fraction of manure slurry as this necessitates two different types of manure handling equipment, storage and land application equipment, to handle both the liquid and solid fractions.
More recently, Dague in U.S. Pat. No. 5,185,079 demonstrated the potential of using an anaerobic sequencing batch reactor (ASBR) for the treatment of swine manure. Dague indicated that the sequencing batch reactor is highly suitable for anaerobic digestion because: 1) It provides quiescent settling conditions for the anaerobic bacteria; and 2) The high food to microorganism ratio (F/M) at the beginning of the feed period and the low (F/M) at the end of the react period enhances the sludge settling characteristics. Dague's ASBR is run in four separate phases; feed, react, settle and decant. In the feed phase, the reactor is fed with the swine manure until the reactor is full which takes about 15 minutes. The manure is then allowed to react with the microorganisms in the reactor to allow conversion of the organic waste to biogas. Dague's react phase generally lasts 6 hours at 25.degree. C. to 35.degree. C. after which the contents of the reactor are allowed to settle (1.5 hours) before the effluent is decanted. One of the drawbacks of Dague's system is that ammonia levels of greater than 1500 mg/L (in swine manure slurry) are toxic to the anaerobic process. Consequently, the manure must be diluted before it is fed to the ASBR to decrease the ammonia levels. The means that the manure must be both diluted and analyzed before it is fed into the reactor, and the ammonia level monitored throughout. Further, the dilution of the manure, which is generally in the order of a factor of four, means that the manure storage facility must also be enlarged by the same factor and a substantially large volume of water is contaminated. As a result, dilution of swine manure will substantially increase the cost of storage structures and land applications of the manure slurry which is neither economically feasible nor environmentally acceptable.